Platinum tip ECP sensor and fabrication thereof

ABSTRACT

A method for fabricating an electrode having extended operating life comprising brazing a cap and adapter to an insulator composed of magnesium oxide-stabilized zirconia (MSZ), calcium oxide stabilized zirconia (CSZ) or yttroim oxide-stabilized (YSZ). Electrodes comprising insulators composed of CSZ, MSZ and/or YSZ are also described.

This application claims the benefit of U.S. Provisional Application No.60/070,096, filed Dec. 31, 1997.

FIELD OF THE INVENTION

The present invention relates to electrodes, especially platinum tip ECPsensors having improved sensor life, and methods for their fabrication.

BACKGROUND OF THE INVENTION

Nuclear reactors are used in central-station electric power generation,research and propulsion. A reactor pressure vessel contains the reactorcoolant, i.e. water, which removes heat from the nuclear core.Respective piping circuits carry the heated water or steam to the steamgenerators or turbines and carry circulated water or feedwater back tothe vessel. Operating pressures and temperatures for the reactorpressure vessel are about 7 MPa and 288° C. for a boiling water reactor(BWR), and about 15 MPa and 320° C. for a pressurized water reactor(PWR). The materials used in both BWRs and PWRs must withstand variousloading, environmental and radiation conditions.

Some of the materials exposed to high-temperature water include carbonsteel, alloy steel, stainless steel, nickel-based, cobalt-based andzirconium-based alloys. Despite careful selection and treatment of thesematerials for use in water reactors, corrosion occurs in the materialsexposed to the high-temperature water. Such corrosion contributes to avariety of problems, e.g., stress corrosion cracking, crevice corrosionerosion corrosion, sticking of pressure relief valves and buildup of thegamma radiation-emitting Co-60 isotope.

Stress corrosion cracking (SCC) is a known phenomenon occurring inreactor components, such as structural members, piping, fasteners andwelds exposed to high-temperature water. As used herein, SCC refers tocracking propagated by static or dynamic tensile stressing incombination with corrosion at the crack tip. The reactor components aresubject to a variety of stresses associated with, e.g., differences inthermal expansion, the operating pressure needed for the containment ofthe reactor cooling water, and other sources such as residual stressfrom welding, cold working and other asymmetric metal treatments. Inaddition, water chemistry, welding, heat treatment, and radiation canincrease the susceptibility of metal in a component to SCC.

It is well known that SCC occurs at higher rates when oxygen is presentin the reactor water in concentrations of about 5 ppb (parts perbillion) or greater. SCC is further increased in a high radiation fluxwhere oxidizing species, such as oxygen, hydrogen peroxide, andshort-lived radicals, are produced from radiolytic decomposition of thereactor water. Such oxidizing species increase the electrochemicalcorrosion potential (ECP) of metals. Electrochemical corrosion is causedby a flow of electrons from anodic to cathodic areas on metallicsurfaces. The ECP is a measure of the thermodynamic tendency forcorrosion phenomena to occur, and is a fundamental parameter indetermining rates of, e.g., SCC, corrosion fatigue, corrosion filmthickening, and general corrosion.

In a BWR, the radiolysis of the primary water coolant in the reactorcore causes the net decomposition of a small fraction of the water tothe chemical products H₂, H₂O₂, O₂ and oxidizing and reducing radicals.For steady-state operating conditions, equilibrium concentrations of O₂,H₂O₂, and H₂ are established in both the water which is recirculated andthe steam going to the turbine. This concentration Of O₂, H₂O₂, and H₂is oxidizing and results in conditions that can promote intergranularstress corrosion cracking (IGSCC) of susceptible materials ofconstruction. One method employed to mitigate IGSCC of susceptiblematerial is the application of hydrogen water chemistry (HWC), wherebythe oxidizing nature of the BWR environment is modified to a morereducing condition. This effect is achieved by adding hydrogen gas tothe reactor feedwater. When the hydrogen reaches the reactor vessel, itreacts with the radiolytically formed oxidizing species to reform water,thereby lowering the concentration of dissolved oxidizing species in thewater in the vicinity of metal surfaces. The rate of these recombinationreactions is dependent on local radiation fields, water flow rates andother variables.

The injected hydrogen reduces the level of oxidizing species in thewater, such as dissolved oxygen, and as a result lowers the ECP ofmetals in the water. However, factors such as variations in water flowrates and the time or intensity of exposure to neutron or gammaradiation result in the production of oxidizing species at differentlevels in different reactors. Thus, varying amounts of hydrogen havebeen required to reduce the level of oxidizing species sufficiently tomaintain the ECP below a critical potential required for protection fromIGSCC in high-temperature water. As used herein, the term “criticalpotential” means a corrosion potential at or below a range of values ofabout −0.230 to −0.300 V based on the standard hydrogen electrode (SHE)scale. IGSCC proceeds at an accelerated rate in systems in which the ECPis above the critical potential, and at a substantially lower or zerorate in systems in which the ECP is below the critical potential. Watercontaining oxidizing species such as oxygen increases the ECP of metalsexposed to the water above the critical potential, whereas water withlittle or no oxidizing species present results in an ECP below thecritical potential.

It has been shown that IGSCC of Type 304 stainless steel (composition inweight % 18.0-20.0 Cr, 8.0-10.0 Ni, 2.00 Mn, 1.0 Si, 0.08 C, 0.08 S,0.045 P) used in BWRs can be mitigated by reducing the ECP of thestainless steel to values below −0.230 V(SHE). An effective method ofachieving this objective is to use HWC. However, high hydrogenadditions, e.g., of about 200 ppb or greater, that may be required toreduce the ECP below the critical potential, can result in a higherradiation level in the steam-driven turbine section from incorporationof the short-lived N-16 species in the steam. For most BWRs, the amountof hydrogen addition required to provide mitigation of IGSCC of pressurevessel internal components results in an increase in the main steam lineradiation monitor by a factor of five. This increase in main steam lineradiation can cause high, even unacceptable, environmental dose ratesthat can require expensive investments in shielding and radiationexposure control. Thus, recent investigations have focused on usingminimum levels of hydrogen to achieve the benefits of HWC with minimumincrease in the main steam radiation dose rates.

In order to evaluate or predict a material's performance at operatingconditions of a reactor, it is important to know the ECP values of thevarious structured components. Electrochemical potential monitoring istypically carried out employing paired electrochemical half-cell probesor electrodes which are mounted within the recirculation piping andaccessed to the external environment through gland-type mountings or thelike. Where the electrode system of interest involves a metal-metal ioncouple, the reference electrode may conveniently be a metal-metalinsoluble salt-anion electrode. A suitable reference electrode may bebased, for example, on the half-cell reaction between silver and silverchloride. Calibration of the cell defining electrode pair is carried outby appropriate Nernst-based electrochemical calculations, as well asthermodynamic evaluation in combination with laboratory testing within asimulated environment against a standard electrode.

A reference electrode which is currently employed is a platinum tipreference electrode for monitoring the ECP. According to conventionaldesign, a platinum cap is brazed to a metallized sapphire insulator postusing pure silver braze. However, the metallizing process is notconsistently reproducible and, in many cases, results in early failure.The silver braze forms an alloy with the platinum cap at high brazingtemperature which, in turn, causes delamination of the metallized layerfrom the sapphire, which is a further reason for early life failure. Inaddition, it has been observed that the sapphire dissolves or undergoescorrosion more rapidly in high velocity water which causes mid-lifefailure.

A need exists for an improved electrode structure with enhancedoperating life for use in measuring ECP. A particular objective is toincrease the sensor life to at least one fuel cycle of the reactor. Thepresent invention seeks to satisfy that need.

SUMMARY OF THE INVENTION

It has been discovered, according to the present invention, that it ispossible to extend the operating life of an electrode by employing analkaline earth metal or rare earth metal-stabilized zirconia ceramic forthe insulator. More particularly, the operating life of the electrodemay be extended by using a calcia-stabilized zirconia (CaO—ZrO₂hereinafter referred to as CSZ), magnesia-stabilized zirconia(MgO—ZrO₂—herein referred to as MSZ) or yttria-stabilized zirconia(Y₂O₃—ZrO₂—herein referred to as YSZ) ceramic for the insulator. It hasbeen found surprisingly, according to the present invention that thedissolution rate of magnesia-stabilized zirconia, calcia stabilizedzirconia or yttria-stabilized zirconia is 10 to 50 times less than thatof sapphire.

According to a first aspect, the present invention provides a method offabricating an electrode including an MSZ, CSZ or YSZ insulator with acap at one end thereof and an adapter at the other end thereof,comprising brazing the cap and adapter to the MSZ, CSZ or YSZ insulatorusing an active metal alloy braze. The resulting brazed assembly exceptthe cap may then be masked with a suitable masking material, such asTeflon ® tubing, and the exposed cap is ion implanted according toconventional techniques at an elevated temperature under vacuum withplatinum to provide a layer of platinum deposited on the cap. As afurther optional step, the cap and adapter are masked with metal foil,and the exposed insulator region is plasma sprayed with a fine powder ofMSZ, CSZ or YSZ at elevated temperature, typically in the region of200-700° C. (deposition temperature) to provide a layer of MSZ, CSZ orYSZ on the sensor and the braze joint.

According to a further aspect, an outer platinum cap (sheath) in a firststep is brazed to an electrode cap using a first active braze alloy. Theresulting outer platinum cap/inner cap assembly together with an adapterare then brazed to a MSZ, CSZ or YSZ ceramic insulator using a secondlower temperature active metal braze. The active braze alloy used in thefirst step should have a melting temperature which is higher than thebraze used in the second step. As an optional third step, the cap andadapter are masked and the exposed insulator area is plasma sprayed witha fine powder of MSZ, CSZ or YSZ at elevated temperature, typically inthe region of 500-700° C. to provide a layer of MSZ, CSZ or YSZ on thesensor and the braze joint.

According to another aspect, the present invention provides an electrodesuitable for use within the environment of a reactor core of a nuclearpower facility fabricated according to the methods of the invention.

According to a further aspect, there is provided an electrode comprisinga housing, a cap and an insulator braze jointed to the cap and housing.The insulator comprises an alloy selected from calcium oxide-stabilizedzirconia (CSZ), magnesium oxide-stabilized zirconia (MSZ) and yttriumoxide-stabilized zirconia (YSZ). MSZ, CSZ and YSZ typically have thefollowing compositions: MSZ: ZrO₂ with 4-8 wt % MgO; CSZ: ZrO₂ with 4-8wt % CaO; YSZ: ZrO₂ with 6-10 wt % Y₂O₃. The cap usually comprises aninner cap of alloy 42 and an outer platinum cap. In addition, a coatingof MSZ, CSZ or YSZ may be provided over an exterior surface of theinsulator which covers the braze joints.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-section of an electrode showing the locationof the ceramic to metal active braze joints;

FIG. 2 is a partial cross-section of an electrode showing a plasma spraybarrier coat;

FIG. 3 is a partial cross-section of an electrode including a clad postcap comprising a platinum outer cap brazed to an alloy 42 inner cap;

FIG. 4 is a partial cross-section of an electrode including a clad postcap and a plasma spray barrier coat;

FIGS. 5a-5 c is a schematic showing fabrication of an electrode of theinvention according to a first method;

FIGS. 6a-6 c is a schematic showing fabrication of an electrode of theinvention according to a second method;

FIG. 7 is a plot showing the expected concentration profile of ionplated Pt on Alloy 42 substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1-4, there is shown an electrode, generallyreferenced 2, having a housing 4 typically of stainless steel, with afirst and second ends 6,8. A stainless steel sheathed mineral insulatedsignal cable 10 is mounted on the second end 8. The first end 6 isconnected to adapter 12 fabricated typically of alloy 42 (alloy 42 is alow expansion iron based on alloy with about 42 wt % Ni and about 58 wt% Fe). The adapter 12 has a necked portion 14 which terminates in amouth 16. An insulator 18 is located in the mouth 16 and is bonded tothe mouth by a braze joint 20. The insulator 18 comprises an alloyselected from calcium oxide-stabilized zirconia, magnesiumoxide-stabilized zirconia and yttrium oxide-stabilized zirconia. A postwire 22 extends longitudinally through the adapter and housing and isconnected to the signal cable at wire connection point 24. The insulator18 and is brazed to a post cap 26 at braze joint 28. The post cap istypically fabricated from alloy 42 and is ion implanted with platinum.The post wire is brazed to an interior surface of the post cap at brazejoint 30.

In FIG. 2, like numerals refer to like parts as shown in FIG. 1. Theelectrode in FIG. 2 further includes a plasma spray barrier coat 32 onan exterior surface of the insulator 18 and extending down over thenecked portion 14 of the adapter 12.

In FIG. 3, like numerals refer to like parts as shown in FIG. 1. Theelectrode in FIG. 3 further includes a clad post cap 34 including aplatinum outer cap brazed to an alloy 42 inner cap 26.

In FIG. 4, like numerals refer to like parts as shown in FIG. 1. Theelectrode in FIG. 4 includes plasma spray barrier coat 32 on an exteriorsurface of the insulator 18 extending down over the necked portion 14 ofthe adapter 12, and clad post cap 34 including a platinum outer capbrazed to an alloy 42 inner cap 26.

Referring to FIGS. 5(a)-(c), a first method of the invention forfabricating an electrode/sensor 2 comprises brazing, typically in asingle step, post cap 26 and adapter 12 to insulator 18 (FIG. 5(a)). Theinsulator 18 is formed of zirconia (ZrO₂) stabilized with magnesiumoxide (MgO) (MSZ), calcium oxide (CaO) (CSZ) and/or yttrium oxide (Y₂O₃)(YSZ). Typical elemental compositions of these zirconia alloys are setforth below:

MSZ: ZrO₂ with 4-8 wt % MgO

CSZ: ZrO₂ with 4-8 wt % CaO

YSZ: ZrO₂ with 6-10 wt % Y₂O₃

The cap and adapter are brazed to the insulator using active metal alloybraze. This braze typically has the following composition:

80-90 wt % Ni; 5-8 wt % Cr; 2-4 wt % B; 3-4 wt % Si

(Braze Temperature: 980-1040° C.).

In the second step (FIG. 5(b)), the brazed assembly comprising theadapter and post cap brazed to the ceramic insulator 18 is masked,suitably with 20 mil Teflon® tubing 36. The cap 26 is then ion implantedwith platinum under a vacuum at an elevated temperature, typically inthe region of 75-100° C. The ion energies employed use the range from 2to 4 Kev. to a depth of about 100 Angstrom. The ion implantation iscontinued until a layer of pure platinum is deposited to a thickness inthe region of 2,000 to 3,000 Angstrom.

A third step may then be carried out (FIG. 5(c)) in which the post cap26 and adapter 12 are masked with metal foil 38. The exposed insulatorregion is then plasma sprayed with a fine powder 40, typically 5-250microns, more usually 5-125 microns of CSZ, MSZ and/or YSZ at anelevated temperature, typically at about 600-700° C. The averageparticle temperature during the deposition ranges from about 2500-3300°C., more usually around 2800-3200° C., depending on the particle size(higher temperatures are typically associated with smaller particles).

FIGS. 6(a)-(c) illustrate a second method according to the presentinvention. In FIG. 6(a), cap 26 is provided with platinum outer cap 34and the two are brazed together at 27 with a first active braze alloy.In FIG. 6(b), the brazed assembly of cap 26 and platinum outer cap 34are brazed at 29 using a second lower temperature active braze alloy tothe ceramic insulator 18, and the adapter 12 is brazed to the insulatorat 31 using the same lower temperature active braze alloy. Thecompositions of the first and second active braze alloys are set forthbelow:

High Temperature (ABA): 80-90 wt % Ni; 5-8 wt % Cr; 2-4 wt % B; 3-4 wt %Si

(Braze Temperature: 980-1040° C.)

Low Temperature (ABA): 58-72 wt % Ag; 26-28 wt % Cu; 2-4 wt % Ti;

(Braze Temperature: 920-960° C.)

The liquidus of the active braze alloy used in the second step isusually at least 60-80° C. lower than the solidus of the active brazealloy employed in the first step.

As an optional third step (FIG. 6(c)), the post cap and adapter aremasked with mask 42 and the assembly is plasma sprayed with CSZ, MSZand/or YSZ powder 44, at an elevated temperature, typically in theregion of 500-700° C. The resultant coating covers the insulator andbraze joints 29, 31 and provides corrosion protection to the brazejoints. In addition, the solid ceramic layer acts as a thermal barrierand provides protection against thermal shock by moderating against anytemperature transients which may occur in the surrounding environment.This minimizes the magnitude of thermal shock(s) in the metal to ceramicbraze joint(s).

Evaluation of the braze joints for leaks with sensor assembliesfabricated using either of the above-described methods was carried outusing standard procedures. The joints passed the standard leak test ofless than 10⁻⁹ stdcc/hr He.

FIG. 7 is a plot of expected concentration profile of ion platedplatinum on Alloy 42 substrate. The platinum was deposited by ionimplantation technique in order to allow the platinum to diffuse intothe surface and form a strong metallic bond with the substrate. This hasbeen qualitatively verified by Auger electron spectroscopy.

The life-span of current platinum ECP sensors is about three to ninemonths in BWR applications. The sensors produced according to thepresent invention exhibit extended sensor life to at least one fuelcycle.

The foregoing has been disclosed for the purpose of illustration only.Variations and modifications of the disclosed methods and structureswill be readily apparent to practitioners skilled in the art. All suchvariations and modifications are intended to be encompassed by theclaims set forth hereinafter.

What is claimed is:
 1. An electrode comprising a housing, a cap and aninsulator braze jointed to said cap and housing, said insulatorcomprising an alloy selected from the group consisting of calciumoxide-stabilized zirconia (CSZ), magnesium oxide-stabilized zirconia(MSZ) and yttrium oxide-stabilized zirconia (YSZ).
 2. An electrodeaccording to claim 1, wherein said MSZ, CSZ and YSZ have the followingcompositions: MSZ: ZrO₂ with 4-8 wt % MgO CSZ: ZrO₂ with 4-8 wt % CaOYSZ: ZrO₂ with 6-10 wt % Y₂O₃.
 3. An electrode according to claim 1,wherein said cap comprises an inner cap of alloy 42 and an outerplatinum cap.
 4. An electrode according to claim 1, and furtherincluding a coating of MSZ, CSZ or YSZ over an exterior surface of saidinsulator and covering said braze joints.
 5. A method of fabricating anelectrode comprising brazing a cap and an adapter to an insulator, saidinsulator comprising an alloy selected from the group consisting ofcalcium oxide-stabilized zirconia (CSZ), magnesium oxide-stabilizedzirconia (MSZ) and yttrium oxide-stabilized zirconia (YSZ).
 6. A methodaccording to claim 1 comprising the farther step of subjecting said capto platinum ion implantation.
 7. A method according to claim 6, whereinsaid adapter and cap are masked and said insulator is subjected to aplasma spray of a powder selected from the group consisting of calciumoxide-stabilized zirconia (CSZ), magnesium oxide-stabilized zirconia(MSZ) and yttrium oxide-stabilized zirconia (YSZ).
 8. A method accordingto claim 1 wherein said MSZ, CSZ and YSZ have the followingcompositions: MSZ: ZrO₂ with 4-8 wt % MgO CSZ: ZrO₂ with 4-8 wt % CaOYSZ: ZrO₂ with 6-10 wt % Y₂O₃.
 9. A method of fabricating an electrode,comprising steps of: (a) brazing a platinum outer cap to an inner capwith a first metal braze to produce a brazed assembly; and (b) brazingsaid brazed assembly to a ceramic insulator using a second lowertemperature metal braze, said ceramic insulator comprising an alloyselected from the group consisting of calcium oxide-stabilized zirconia,magnesium oxide-stabilized zirconia and yttrium oxide-stabilizedzirconia.
 10. A method according to claim 8, wherein said second metalbraze has a melting point at least 60-80° C. lower than said first brazealloy.
 11. A method according to claim 8, wherein the cap and adapterare masked to form an exposed region which is plasma sprayed with apowder selected from the group consisting of calcium oxide-stabilizedzirconia, magnesium oxide-stabilized zirconia and yttriumoxide-stabilized zirconia at a temperature of 500-700° C.
 12. A methodaccording to claim 8 wherein said first metal braze has a composition of80-90 wt % Ni; 5-8 wt % Cr; 2-4 wt % B; and 3-4 wt % Si.
 13. A methodaccording to claim 8 wherein said second metal braze has a compositionof 58-72 wt % Ag; 26-28 wt % Cu; and 2-4 wt % Ti.